Engine chunking noise after partial rebuild
 

Guys-

I realize its nearly impossible to diagnose an engine sound over the internet, so Id like to give me some advice on where I should look. Here is the situation:

This winter I replaced the gaskets in the chain boxes of my SC motor because of oil leaks. I have an '83 with the bolt style cams. Re-timing went well, I checked and double checked the setup, the engine turned over by hand without a problem. Engine went in fine, started fine, and idles without a hitch. The only exception is that there a really loud chunking noise coming from near cylinder #4. Say "chunk, chunk, chunk" about as fast as you can and that is about the pace of the sound. It increases in speed with the engine RPMs.

I never had this noise when I took the engine out so Im not convinced its a rod knock (but what do I know :) ). I also don't hear it when I crank the engine over with the starter and no spark. We're going to do a compression test ASAP, but Im curious if anyone else has had this issue. All my tools are accounted for so I don't think I left anything down in the engine. Anybody had something like this before?

I would use a stethascope or long drift to isolate the noise. Sound like one side or the other. You mentioned #4. Did a rocker come loose? Have you pulled the valve cover for a looksee?

oh, the pain of being away from home and having to leave it sitting knowing there is a knock. I feel you pain. I got the last of my car put together this weekend, and did a few "dyno pulls" up my driveway. I was thinking of you as I was setting the timing, knowing you were out for the week and had this porblem. I wondered if you would post or not until you had a chance to look around more.

The Rocker coming loose is a good suggestion, also how sure are you about the alignment of the chain sprockets, I would have guessed that becuse you did not disturb the cam housings you should have had the same number of shims, but did you check it as well? It is possible that the chain is hanging up on the sprocket and poping into place as it rotates, like a bicycle does as you shift gears.

Plus is the clunk time consistant with the rpm?

Jim

exhaust gasket
air pump port open

is the noise louder top side or bottom side?

Fantastic! Im thinking its a rocker, that makes complete sense.

Its louder on the top of the engine
The sound is dependent on the rpm (more rpm = faster chunk).
Much too loud to be a gas leak.
The cam gears went in just fine, no left over washers.

Thanks a bunch guys! Im going to be checking the rockers as soon as I get home on Friday. If you have any more suggestions please let me know.

I second rocker. Sounds like a problem I had. My guess would be the intake side. The exhaust side noise when the rocker shaft slides out is like the rod failure. I was scared to death when I heard that noise.

Did you torque the rocker shaft bolts properly?

exhaust rocker
remove lower valve cover and check

loose rockers really don't "clunk". more tappy sounding.

But if the rocker has backed out, the exhaust valve does not open, it "chuffs" out the intake side

but he said it idled smoothly. it would have a dead hole if that happened.

ok, i isolated the problem:

The dial guage I used was in inches not in milimeters so the cams weren't timed to anything even close to what they needed to be. (~0.2 instead of in the 0.9 - 1.1 range). I turned the engine over by hand when I set this timing and was certain than the valves weren't hitting, but that was by hand...

So here is where Im at now (and I would really appreciate some feedback on this guys):

I pulled the heads and the cam tower as a full unit off the engine after seeing 0ft/lbs on my compression tester (with the engine cold). #6 registered in with 120ft/lbs, 5 and 4 were dead. I pulled the heat-exchanger and could hear air escaping the cylinders on compression past the exhaust valve.

The valves and pistons don't look bad, but the carbon crust laters on the pistons and valves have obviosly been in contact. This may have occured when I first timed the cams, the system did bind up on a few occasions (by hand ofcourse).

I still don't know if the sound I heard was a valve contacting a piston or if it was just the valve train popping/backfiring/etc from mis-timed cams. Im also not sure if the valves are bent or not. Is there a good way to test them? Would it take the velocity of a running engine to properly seat the valves in order to test them?

So, what would John Walker do, thats what Id like to know?

well, i would take all the valves out and spin them in your drill motor or some other chuck to check if they're bent. exhaust valves tend to bend before the intakes, but that's on an over-rev in a properly timed engine, so best to check them all. bummer!

You will need to know if the valve stems are straight. You will need to disassemble the heads. There is no practicle way around it. To verify a valve job one should pull a vacuum on the intake and exhaust ports.

David Duffield